twisted-dev.el needs better local variable handling

[Jean-Paul Calderone]

Originally reported by jerith (<http://twistedmatrix.com/trac/ticket/5186>).

There are two related things I found myself needing while using twisted-dev.el:

Per-project settings for 'twisted-dev-directory' and 'twisted-dev-scratch-directory'
A 'safe-local-variable' setting for 'test-case-name'
Both of these can be done outside twisted-dev.el, but I think they're useful enough to add.

The attached patch works for me in Aquamacs (based on emacs 23) and should be fine for other emacsen as well.